The present invention concerns a rotor for a centrifugal separator. This rotor is arranged to rotate during operation around a rotational axis in a space in the centrifugal separator in contact along its outside with a gas present in the space. Inside itself the rotor forms at least one chamber and at least one passage, which with a central axis extends radially outwardly from the chamber to the outside of the rotor and connects the chamber with the space outside the rotor. The chamber and the passage are arranged so that, normally intermittently during operation, a component separated in the rotor flows therethrough.
A device of this kind is described e.g. in US-A-5 202 024.
In order to avoid the chamber and the passage becoming clogged and to avoid the through-flowing separated component causing unacceptable wear of the rotor body by erosion, the chamber and the passage in the rotors of many known centrifugal separators have a complicated shape, which is produced by an expensive disc mill cutting operation on both the inside and the outside of the rotor body.
During operation rotors of this kind furthermore generate sound, which in many cases exceeds the sound level which can be accepted. This is particularly the case for rotors which rotate at a relatively high number of revolutions. Often a significant part of the sound is generated in the above mentioned chambers and passages. This sound is generated by the gas, usually in the form of air, which is located in the space, when during operation of the rotor, it passes the openings towards the space of the passages. The frequencies of the generated sound, the pressure waves, is determined by the geometric shape and dimensions of the chamber and the passages.
In DE 1 298 449 a simpler design of passages of this kind in a centrifugal separator is shown. In the rotor known hereby a number of passages in the shape of circular cylindrical holes, which are distributed around the rotational axis and radially directed, connects a chamber, which is formed inside the rotor and surrounds the rotational axis, with the space outside the rotor. By designing the chamber and the passages in this manner the manufacturing cost can be reduced considerably.
However, it is very easy for gas, which is located in circular cylindrical holes of this kind, to be excited in the same manner as the gas in an organ pipe by a gas, passing by the opening of the hole and be set in oscillation and thereby generate sound at a certain frequency.
The object of the present invention is to accomplish a rotor of the kind initially described at low manufacturing costs for a centrifugal separator having a chamber and at least one passage, which are so designed that a separated component can flow through the same without a risk of clogging or wear and so that gas, which during operation is located in the chamber and the passage, is not set in oscillation and does not generate unacceptable noise levels.
This is accomplished by the present invention by designing the passage with a radially outer portion, which extends radially inwardly from the outside of the rotor, and which perpendicular to the central axis of the passage has a cross-section, which in a plane perpendicular to the rotation axis and containing the central axis has an extension which continuously decreases over the distance from the outside of the rotor to a radially inner cross-section, which has a certain extension in this plane and from the outside of the rotor is located at a distance along the central axis of at least one third of the extension in said plane of the radial inner cross-section, and that in the plane the extension of the cross-section of the passage, at the outside of the rotor, is between four thirds and nine thirds of the extension of the radially inner cross-section in said plane.
In a preferred embodiment of the invention the radial outer portion of the passage has a circular cross-section.
The radially outer portion of the passage then can either have a spherical shape with a centre, through which the central axis extends, or a conical shape around an axis of symmetry, which coincides with the central axis.
In a modified embodiment of the invention the radially outer portion of the passage has an elliptic cross-section, the major axis of the ellipse being directed in the circumferential direction.
In another embodiment of the invention the passage in the plane perpendicular to the rotational axis and containing the central axis, has an extension perpendicular to the central axis which decreases continuously radially inwardly from the outside of the rotor on both sides of the central axis.
In a further embodiment of the invention the passage also comprises a radial inner circular cylindrical portion, which is located radially inside the radially outer portion of the passage.
In a suitable embodiment of the invention the passage is straight and directed completely radially, but as an alternative it can be straight and radially outwardly seen be directed backwardly with respect to the rotation direction.
In the following the invention will be described more closely with reference to the attached drawings, in which
The centrifugal separator shown in figure 1 has a rotor 1, which is supported by and driven by a driving shaft 2. The rotor 1 is surrounded by a stationary casing 3, which forms a space 4, in which the rotor 1 during operation rotates in contact with the air or other gas, which is located in the space 4, along its outside. Inside itself the rotor 1 forms a separation chamber 5, in which a stack of separation discs 6 is arranged. Between these separation discs 6 the main centrifugal separation during operation takes place, a specific light component, which is separated out of a supplied liquid mixture of components flows radially inwardly while a specific heavier separated component in the form of a liquid or a sludge flows radially outwardly and is accumulated in the radially outermost part of the separation chamber 5.
When needed or at equal time periods all or part of the content of the separation chamber 5 is thrown out through a passage 7, which extends with a central axis radially through the wall 8 of the rotor 1, by opening an annular gap 9 surrounding the rotational axis between a rotor cover 10 and an axially movable valve slide 11. Between the gap 9 and the passages 7 there is a chamber 12, which is delimited by the rotor wall 8, the rotor cover 10 and the valve slide 11.
The passage 7 has a radially outer portion 13, which extends radially inwardly from the outside of the rotor 1. Perpendicular to the central axis of the passage this outer portion 13 has a cross-section which, in a plane perpendicular to the rotational axis and containing the central axis, has an extension which continuously decreases over a distance from the outside of the rotor 1 to a radially inner cross-section. This radially inner cross-section has a certain extension in the plane and is located at a distance along the radially directed central axis from the outside of the rotor 1, which at least is one third of the extension of the inner cross-section in this plane. At the outside of the rotor the extension of the cross-section of the passage 7 in the plane is five thirds of the extension of the radially inner cross-section.
In the example shown in figure 1 of a rotor 1 according to the invention the passage also comprises a radially inner portion 14, which connects to the radially outer portion 13 at said radially inner cross-section from where it extends radially inwardly to the chamber 12 in a way such that this chamber 12 communicates with the space 4 outside the rotor 1 via the passage 7. The radial inner portion 14 is circular cylindrical while the radial outer portion 13 in this example is frusto-conical with the apex directed radially inwardly. The central axis of the passage 7 is straight and extends completely radially in the rotor.
Figure 2 shows in more detail the passage 7 shown in figure 1 seen in a radial section through the central axis of it.
Figure 3 shows another example of how a passage 15 can be designed seen in a radial section through its central axis, which also in this example is straight and extends totally radially in the rotor. This passage 15 has a spherical radially outer portion 16 and a circular cylindrical radially inner portion 17.
In the example shown in figure 4 of a design of a passage 18 seen in a radial section through its central axis the central axis is straight but seen radially outwardly is directed backwardly with respect to the rotational direction, which is indicated with an arrow A. As the passage 7 shown in figures 1 and 2, this passage 18 has a radially outer frusto-conical portion 19 and a radially inner circular cylindrical portion 20.
Figure 5 shows a view radially from the outside of a modified passage 21, which is designed with a radially outer portion 22 with an elliptic cross-section, the major axis of the ellipse being oriented in the circumferential direction.
In order to illustrate the technical effect of the present invention the figures 6-8 show diagrams of the measured frequency spectrums of the sound levels for three different designs of passages of this kind.
In figure 6 there is shown the frequency spectrum of the sound level for circular cylindrical passages without any radially outer portion of the kind with which the passages in a rotor according to the present invention are provided as described above.
In figure 7 there is shown a frequency spectrum of the sound level for circular cylindrical passages, which in the same way as the passage in the rotor according to the present invention has a radially outer portion, which extends radially inwardly from the outside of the rotor, and perpendicular to the central axis of the passage has a cross-section, which in a plane perpendicular to the rotational axis and containing the central axis has an extension, which continuously decreases over the distance from the outside of the rotor to a radially inner cross-section, which has a certain extension in this plane but is located from the outside of the rotor at a distance along the central axis shorter than one third of the extension of the radially inner cross-section in said plane.
From these two diagrams it is evident that the generated sound has a basic background noise level, to which a number of excited tones are added. From the diagram in figure 7 it is also evident, that tones of this kind are excited and added to the background noise level even if the passage or the passages are provided with a short radially outer portion of this kind.
The diagram in figure 8 shows that if a passage is designed in the way a rotor passage according to the present invention is so that the radial cross-section of the passage is located at a distance from the outside of the rotor, which is at least one third of the extension of the radial inner cross-section in the plane perpendicular to the rotational axis and containing the central axis, the excitation of tones are reduced so substantially that only the background noise level of the sound remains.